The performance of aircraft tail surfaces, particularly the horizontal tail plane and the vertical tail plane, is one of the more important issues in global aircraft design because said surfaces are used as control surfaces that must provide stabilizing forces even at high angles of attack to restore the aircraft attitude.
Tail stall angle is a design constraint related to the safety of the aircraft flight and is determined by the taper and aspect ratio of the surface as well as, among other design features, the aerofoil thickness and leading edge shape, so that the aircraft industry is constantly demanding new designs of tail surfaces that allow delaying stall, particularly in icing conditions.
Regarding wings, U.S. Pat. No. 6,431,498 discloses an apparatus to modify a wing to provide increased lift over drag ratios compared to similar wings with straight leading edges forming a plurality of protrusions (inspired on the tubercles on the leading edges of humpback whale flippers) spaced laterally along the leading edge, the protrusions creating a smoothly varying, alternately forward-and-aft sweep along the leading edge relative to the upstream flow direction along the leading edge. One of the effects of said modification is the delay of stall at high angles of attack.
The maximum lift coefficient of wings is an important consideration for the design of wings and there are very effective high lift devices used in the aircraft industry to increase the wing lift coefficient in order to reduce the stall speed enabling the safe flight at low speeds. Trailing edge devices, like flaps, produce an increment of lift coefficient while maintaining the same angle of attack of the wing. Leading edge devices, like slats, droop noses, dog-teeth, serrated leading edges and aerodynamic fences, enable an increase of the stall angle and therefore of the ultimate lift coefficient. A crucial consideration in the design of wings is to reduce the drag in cruise configuration so it is desirable that whatever high lift devices are used cause a small drag increase. Movable leading edge devices, like slats and droop noses can be retracted so that the wing aerofoil shape in cruise is not perturbed. Therefore, in cruise condition, the maximum stall angle of the wing corresponds to the “clean wing” configuration, i.e., without high lift devices. Fixed leading edge devices, like dog teeth, fences, etc. . . . , cause a drag increase in cruise and are therefore avoided in the design of high performance wings like those of modern commercial transport aircraft.
When an aircraft encounters a flight situation such that the wing may stall (as a consequence of severe turbulence which may upset the aircraft attitude or in the case of flight through a region of the atmosphere with icing conditions, where ice can be accrued in the wing leading edge, breaking the aerodynamic smoothness of the aerofoil), it is essential that the tail surfaces remain effective in providing sufficient aerodynamic forces to restore the aircraft attitude. An important design requirement for the aircraft tail surfaces is, therefore, that their stall angle is greater than that of the wing, even in icing conditions.
During flight at low speed, where the wing high lift systems are deployed, the wing is liable to stall if the pilot inadvertently flies below the stall speed or performs an unusual maneuver which increases the angle of attack beyond the stall angle of the wing. In this condition, it is essential that the tail surface provides sufficient aerodynamic forces even with the rudder or elevator deflected and, particularly in icing conditions, where there may be ice accrued on the tail leading edge.
It must be noted that in order for the tail surfaces to generate aerodynamic forces in situations where the wing may be stalled, the critical design condition is that the tail stall angle be larger than that of the wing.
It is clear that an essential design requirement for aircraft tail surfaces is the stall angle, this being much more important in the case of tails than in the case of wings, where aerodynamic “finesse” (ratio of lift to drag in cruise) and maximum lift coefficient (at the lowest angle of attack possible, to minimize fuselage drag and risk of tail strike on the ground) are the most important aerodynamic design requirements.
In particular, tail stall angle in icing conditions, when it is assumed that the tail leading edge has an ice form which breaks the aerofoil smoothness and thus reduces the maximum lift coefficient, is a crucial design consideration for modern commercial aircraft. There are several accidents documented where the root cause has been the stall of the tail in icing conditions and therefore the loss of control of the aircraft.
There are known methods to minimize ice accretion on the tail surfaces consisting on heating the leading edge or on having a flexible leading edge which can be inflated with the aim of preventing the formation of ice on the leading edge or breaking the ice once it has formed. The operation of these devices requires a positive action from the pilot who activates them if atmospheric icing conditions are detected. These methods not only are costly to install and maintain but carry the risk of not being operative when required, without prior indication.
Thus it is clear that passive means to prevent ice accretion on the leading edge would be preferred.
The present invention is intended to the attention of said demand.